This invention relates to secondary lithium intercalation batteries and, particularly, to such batteries which comprise a lithium ion source electrode and a carbon electrode. More particularly, the invention relates to the improvement in the reversible intercalation of lithium into carbon electrodes, notably graphite, at high cycling rates and with maintained cell capacity by means of the utilization of a unique class of non-aqueous electrolyte.
The ability of carbon, such as petroleum coke or graphite, to intercalate lithium ions into its structure has been known for some time. A more recent study of such intercalation from a metallic lithium source was reported by Fong et al. (R. Fong, U. Von Sacken, J. R. Dahn, J. Electrochem. Soc., 137, 2009 (1990) and noted, in particular, that when combined with typical non-aqueous electrolyte compositions in a battery cell structure, the lithium/carbon combination encounters a significant initial inability of the carbon electrode to reversibly intercalate lithium ions. The Fong study attributed this irreversible loss of lithium to its assimilation in reactions resulting from the apparent passivation of the carbon electrode surface during the first discharge of the battery cell. Up to about 30% of the total initial capacity was seen to be lost in this irreversible reaction when using a petroleum coke electrode, while the loss of as much as 50% of the total initial capacity of lithium into a graphite electrode was observed. In otherwise desirable secondary cells utilizing safer lithium intercalation compounds as lithium source electrodes in lieu of the more highly reactive lithium metal, such a loss of lithium is disadvantageous, indeed, when one considers the limited store of lithium normally available in the lithiated intercalation positive electrode.
In our application, Ser. No. 07/871,855, filed 21 Apr. 1992, now U.S. Pat. No. 5,192,629, we described a non-aqueous electrolyte for a secondary battery having lithiated intercalation and carbon electrodes which resists destructive oxidation previously encountered during attempts to charge such a battery at the higher voltages necessary to take advantage of improved intercalation materials, such as Li.sub.1+x Mn.sub.2 O.sub.4 (0&lt;x&lt;1). We have now discovered that the irreversible loss of lithium on carbon during the first discharge of the secondary battery, which in the past has detracted from the utility of these carbon electrodes, can be significantly reduced by employing this electrolyte in such intercalation cells. We discovered, also, that use of this electrolyte maintains the high level capacity (Ah/g) of the cells even at increased cycle rates up to the range of about 2C (complete discharge in 30 minutes).